Add a note to the README about our custom GLuaTest extensions

[sarahsturgeon]

The stuff we have in:
https://github.com/CFC-Servers/gmod_tests/tree/main/lua/gluatest/extensions

Is useful, but it should be documented somewhere in the README so folks know they exist.

Further notes can be added about:

• Why we have these
• When you should consider making your own

Additionally, better docs should be added inside each of the extensions to outline what they do, how to use them, good references for existing use cases, etc.